Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and is suitable as an image pickup optical system for use with a photographic apparatus such as a digital still camera, a monitoring camera, or a silver-halide film camera.
Description of the Related Art
In recent years, an image pickup apparatus using a solid-state image pickup element is increased in functionality, and is downsized as the entire apparatus. As an image pickup optical system used with the image pickup apparatus, a zoom lens is required to have a short total lens length, a high zoom ratio, and a high resolution. As a zoom lens satisfying those requirements, for example, a zoom lens using a so-called “rear focus system” in which a lens unit other than the first lens unit arranged closest to an object side is used for focusing has been known.
One reason why the rear focus system is adopted is that, in a lens unit for focusing (hereinafter referred to as “focus lens unit”), a height of incidence h of an on-axis marginal ray is low, and hence an aberration variation (in particular, a variation in spherical aberration) during focusing may be reduced.
Another reason is that, when a size of the image pickup element used in the image pickup apparatus is small, the focus lens unit is reduced in weight and also in amount of extension during focusing, and hence the entire image pickup apparatus becomes easy to downsize. In addition, in recent years, in order to make the image pickup apparatus compact, electronic distortion correction in which, of various aberrations of the zoom lens used with the image pickup apparatus, distortion is electrically corrected in a wide angle range is often introduced. An image pickup apparatus in which electronic distortion aberration correction is introduced is advantageous in making the image pickup apparatus compact. However, a problem in that the aberration variation during focusing becomes large has been known.
In aberration theory, a process of focusing from a long distance to a short distance can be divided into a process in which an object distance moves from the long distance to the short distance (object distance movement) and a process in which the focus lens unit is moved to focus on the object distance (focus lens unit movement). When the aberration theory is used, it can be seen that a variation in field curvature is increased theoretically during the object distance movement, and that a variation in field curvature during focusing also tends to be increased. This is proved based on the fact that, according to Yoshiya Matsui “Lens Design”, Chapter 4, a third-order aberration coefficient V of distortion is large, and hence a variation amount of a third-order aberration coefficient III of astigmatism during the object distance movement:(ΔIII)1=−δ(V+IIs)+δ2Is 
also becomes large (δ represents an object distance movement parameter, IIs represents a third-order aberration coefficient of coma of a pupil, and Is represents a third-order aberration coefficient of spherical aberration of the pupil). However, when the size of the image pickup element is small, the variation in field curvature during focusing is inconspicuous, and the problem is hardly addressed.
For example, in Japanese Patent Application Laid-Open No. 2010-181787, a zoom lens in which a rear focus system is adopted and electronic distortion correction is introduced in a negative lead type including a lens unit having a negative refractive power closest to an object side is disclosed. Moreover, in Japanese Patent Application Laid-Open No. 2010-181543, a zoom lens in which a rear focus system is adopted and electronic distortion correction is introduced in a positive lead type including a lens unit having a positive refractive power closest to an object side is disclosed.
The zoom lens using the rear focus system makes it easy to reduce the size and weight of the focus lens unit, and to focus quickly. However, the aberration variation tends to increase during focusing. Moreover, in general, when the image pickup element used in the image pickup apparatus is increased in size, the zoom lens used with the image pickup apparatus is required to have high optical characteristics over the entire zoom range and the entire object distance. For example, it is required that the aberration variation during focusing be small, and that the variation in field curvature be small in order to maintain the high optical characteristics over the entire image plane.
On the other hand, in the zoom lens used with the image pickup apparatus having a function of performing the electronic distortion correction, an increase in distortion aberration is permitted, and hence it becomes easy to realize a wide angle of view and the downsizing of the zoom lens. However, in such a zoom lens, during focusing in the wide angle range, the variation in field curvature and the variation in spherical aberration are increased.
In general, in the zoom lens, in order to obtain the high optical characteristics over the entire image plane with the small aberration variation during focusing while having a predetermined photographing angle of view, it becomes necessary to appropriately set the zoom type, the refractive power and a lens configuration of the focus lens unit, and the like. Further, it becomes necessary to appropriately set lens configurations and the like of lens units located on the object side of the focus lens unit.